Over the last several years, a type of memory board has come into use called the SIMM (single in-line memory module). There are many types of SIMMs, i.e., 30 pins, 72 pins, and 80 pins. Typically, such modules are used for dynamic random access memories (DRAMs) and have predefined pin-out assignments. At present, the 72 pin SIMM sees the widest use in industry. For each of these SIMMs, an industry standard exists that defines the signals to be found on each pin. Thus, any DRAM that adheres to the SIMM standard and is plugged into a receiving connector, provides at the output pins of the connector, known signal levels and data.
Since DRAMs are ubiquitous in data processing systems, SIMM-connectors see wide usage therein. In such systems, it is also the case that other types of memory modules are used, although not in such large quantities. For instance, such other memory types include read only memories, electrically erasable, programmable read only memories; and battery-backed static random access memories. Additional expense is required if special connectors must be provided to accommodate the different types of memory modules. In addition, there are also certain DRAMs which do not meet the SIMM pin-out standard and also require special connector sockets.
In one industry standard for 72 pin DRAM SIMMs, four output pins are assigned to a "presence detect" function. When such a SIMM is plugged into a connector on a mother board, its presence detect pins are connected, via a resistance, to a power supply. Two of the four pins indicate one of four logic states which tells the user that the memory is one of four predefined sizes. The other two pins use three logic states to define one of three speeds. The fourth logic state, i.e., both speed pins at the high or one state, tells the user that the SIMM connector is empty. If a SIMM connector is to be utilized for other types of memory modules, it is important that the presence detect pins retain their standard output states so as to enable their continued usage for standard SIMM DRAMs, in addition to other memory modules.
In addition to the four-pin presence detect standard, others have employed various methods and means for determining the configuration of modular memories. DeVoy et al., in U.S. Pat. No. 3,803,560 describe a modular memory system which, in response to command signals, removes modules detected as 7 faulty and reconfigures the remaining modules to form a continuous address space. In U.S. Patent RE 31,318 to Kaufman et al., a system is described for automatically setting the address ranges of memory modules in a continuous bank of memory modules. Each module includes an address range calculator, an address range detector, a local memory unit and memory cell selection logic. In operation, a processor generates a starting address signal for a first installed memory module. The address range for an individual memory module is calculated by adding the local memory unit capacity to the module's starting address to arrive at the module's ending address. This action ripples through additional memory modules until the last memory module generates an address signal which represents the upper boundary of the memory system.
A further memory allocation system is described in U.S. patent application Ser. No. 07/578,699 to Cox, assigned to the same assignee as this application. The Cox Application describes an expandable memory system that employs a plurality of plug-in memory modules. Each memory module includes a controller that is serially linked to a central memory controller. The memory system is automatically configured by the central controller and each memory module is assigned a base address which, in turn, helps to define a contiguous memory space and requires no user intervention. The system has the capability of disabling and bypassing bad memory modules and reassigning memory addresses without leaving usable memory unallocated.
Accordingly, it is an object of this invention to provide a modular memory arrangement wherein standard SIMM connectors are useable by all memory modules including DRAMs.
It is another object of this invention to provide a modular memory system wherein a memory module, upon being addressed, automatically provides information detailing the module's characteristics.
It is yet another object of this invention to provide a memory module that does not adhere to a predetermined standard for presence detect output pins, but is yet able to interface with a standard SIMM connector by internal control circuitry.